The present invention generally relates to forming holes in, etching the surface of, or otherwise ablating substrates. The invention particularly relates to processes for etching surfaces of substrates and forming holes having varying diameters along their depth.
Microholes are used in numerous industrial applications, such as but not limited to, orifices in diesel engine fuel injectors and cooling holes in aerospace engines. As used herein, “microholes” are holes with average diameters in the range of ten to a few hundred micrometers (for example, 50 to 500 μm). Microholes of various cross-sectional shapes whose cross-sectional dimensions (for example, diameters or widths) vary along their lengths or depths, defined herein as “non-straight” microholes, maybe desirable in certain situations, for example, when beneficial to product performance. As a particular example, in diesel fuel injectors, non-straight microholes (for example, reverse tapered, i.e., holes having a cross-sectional dimension that increases in the direction of its depth, or converging-diverging holes, i.e., holes having a cross-sectional dimension that decreases and then increases in the direction of its depth) may enhance fuel atomization for a more complete combustion providing higher fuel efficiency and reduced emissions. As another example, non-straight microholes may yield improved cooling performance in gas turbines.
In general, it can be very challenging to drill non-straight microholes. For example, when forming a microhole with a laser beam, the cross-sectional dimensions of the microhole are often such that there is insufficient line-of-sight necessary to deliver the laser beam directly to the microhole sidewall to modify its cross-sectional dimensions in a controlled manner. Attempts have been made to drill reverse tapered microholes through electrical discharge machining (EDM) and micro electrochemical machining (ECM). However, these techniques are limited to conductive materials, as both EDM and ECM rely on the flow of electric current from the tool to the workpiece.
Further, EDM has a difficulty in producing microholes with arbitrary diameter variations (that is, microholes with nonuniform cross sectional dimensions through their depth), such as a converging-diverging hole. EDM and ECM also suffer from low efficiency and high associated costs of consumables. For example, it may take several minutes to drill one microhole using ECM, which has seriously limited its industrial applications.
Thus, there is an ongoing desire for fabrication methods capable of producing non-straight microholes in both conductive and non-conductive substrates.